headers: making low-level headers standalone

This starts shrinking the standalone-header allowlist with a low-risk batch.

What I found:

• ed25519.h, fe25519.h, and string.h were including libssh/priv.h just for fixed-width integer types.
• replacing that with <stdint.h> makes them standalone without dragging in internal platform/config helpers.
• ge25519.h also starts passing because it includes fe25519.h.

The standalone-header check now passes with 17 known failures left. Follow-up MRs can tackle the remaining clusters separately, starting with the crypto/wrapper type dependencies, then the pki/kex/session-related headers.

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated
• The project pipelines timeout is extended at least to 2 hours.

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTING.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code